Add Anima::Defaults

Add a way to specify defaults for certain attributes.

Probably not up to your standards, but the basic idea is there. Had some issues running mutant, this will need some more coverage.
I wounder if we could patch `AnimaInfectedClass.new` to call `super` with the merged defaults.

```
def self.new(attributes)
  super(anima.defaults.merge(attributes))
end
```